Your soul needs to grow to maturity, nourished by the holy sacraments in the life of grace. Once you have completed this initiation rite, you will know also the correct positioning of the fingers and thumb while making the cross as well as the words you must say as you do this.  Also, the cross is formed differently and different words are uttered depending on your denomination.  Improperly forming the cross, saying the wrong words or even this act done correctly but by the uninitiated will likely result in cataclysm or at the very least, damning your immortal soul to hell.  You'd be better off to learn some other gang signs.